1. Technical Field
This invention relates generally to digital ink jet printers, and more specifically to such printers that have narrow printhead segments which produce adjacent bands of printed pixels.
2. Background Art
Printheads narrower than the page width, such as disclosed in U.S. Pat. No. 5,384,587, which issued to Takagi et al. on Jan. 24, 1995, require multiple parallel swaths the printing each image plane, as shown in FIG. 1, wherein a narrow printhead prints one image plane by multiple parallel swaths. The width of the swaths is determined by the width of the printhead. Whereas the narrower printheads have the advantage of lower cost, they are very slow.
The printing speed of digital printers depends on the width of the printhead. A cross-the-page, full width printhead can print an image plane in a single pass, and is therefore most desirable for high speed printing. But full width printheads have the disadvantages of being more difficult and costly to fabricate because a single defect in the head makes an entire head defective. Page-width printers of the continuous ink jet type made from a single array of nozzles are known to the art but have not found use in high quality printing applications due in part to difficulties obtaining a high density of nozzles and to the need for ink recirculation. Page-width ink jet printheads, of the drop on demand type, have the disadvantage of not being cost effective due in large part to difficulties of thermal management and printhead lifetime.
One approach to a full width printhead is to use an array of narrow printhead segments laid out across the page, as shown in FIG. 2. The printhead segments are distributed in a staggered fashion so that the printing areas of the neighboring segments overlap with each other, as shown in FIG. 3. This design saves cost and also allows the flexibility of being able to separately replace each individual printhead segment if one becomes defective. However, "banding" defects often occur at the interface between adjacent printhead segments.
As used herein, the phrase "printing pixel" refers to the printhead structure that effects modulation of the media to cause a printed pixel. The printing pixel may be, for example, a resistive heating element, an ink jet nozzle, or a light source.
Banding is caused by miss-registration between the printhead segments of the array. For example, if adjacent printhead segments overlap by one printing pixel, a dark line occurs in the overlap. Likewise, if there is a one pixel-wide gap between adjacent printhead segments of the array, a line will not be printed, leaving a white line in between the two segments. This problem exists in both continuous tone and halftone, and in different types of digital printers (such as those using resistive thermal, ink jet, laser, and silver halide technologies).
In U.S. Pat. No. 5,384,587, which issued to Takagi et. al., the problem of banding-is recognized for multi-drop ink-jet printing. The patent discloses a method to reduce banding by overlapping the printhead scans. In the overlap region, each media printed pixel receives some drops of ink from a nozzle at one end of the printhead and then on a subsequent scan receives additional drops of ink from a nozzle near the other end of the printhead, the total number of drops ideally equaling the number of drops which would have been received by a single printhead whose scans were not overlapped. By altering the number of drops delivered by nozzles at each end of the printhead from a constant number characteristic of the number of drops delivered by nozzles in the middle of the printhead to a much smaller number at the printhead end, banding in the overlap region is reduced. The number of ink droplets is gradually decreased toward a discharging portion disposed in an edge portion of the recording head. U.S. Pat. No. 5,384,587 also teaches methods whereby multiple drops deposited during subsequent scans can be spaced apart to compensate for spreading effects of multiple ink drops not deposited simultaneously but instead deposited during separate scans, an effect dependent on the ink-paper interactions.
U.S. Pat. No. 4,622,561 also teaches a method to reduce banding by overlapping the printhead scans. Subsequent scans are stepped by 50% of the scan width of a single scan, and the centers of the ink drops deposited during each scan are displaced by one half of one pixel. This method also reduces the sensitivity of banding to accidental displacements of printhead scans and provides uniformity and consistency of dot formation. U.S. Pat. No. 4,999,646 also teaches a method to reduce banding by overlapping the printhead scans by depositing first and second partially overlapping complementary dot patterns displaced by half the final dot-to-dot spacing to promote uniform and consistent drying.
European patent application 0,539,157,A2 by Hirabayashi et al. teaches a method of reduction of color banding during multi-color ink jet printing caused by edge displacement of two colors co-deposited in the same location but at different times. The time delay between deposition of subsequent dots of different colors in the same spatial location produces different banding on each end of the scanned printhead. For cases of multiple printheads, each of which print different colors and which print color mixtures by superposing two dots of complementary colors, the spread of the ink dot last deposited is reduced. This reduction is beneficial near the ends of the printhead, particularly near the leading edge of the scan lines. Alternately, the printhead is displaced during a second scan so that the edge of the second color dot deposited is displaced away from the leading edge of the scan line. The amount of such deliberate displacement of the edge of the second color drop is not large compared to the dot sizes. European patent application 0,539,157,A2 does not teach overlapping scans of similar colors.
The occurrence of banding may be understood quantitatively from a consideration of a printhead actuation function, a printhead transfer function, and a media modulation function. The printhead actuation function describes how printhead printing pixels are actuated across a printhead or a printhead segment. The transfer function describes the extent to which each actuated printing pixel provides media modulation for a given level of activation. The media modulation function, which is approximately the product of the printhead actuation function and the printhead transfer function, describes the resultant modulation by the printhead of the media sheet on which the image is printed. Modulation of the media results in a visible image.
The actuation function applied to the printing pixels of the printhead and the media modulation function applied by each printing pixel of a printhead to the corresponding pixels of the media sheet depend on the type of media and the type of printhead. For example, the actuation function applied to a thermal printhead or to an ink jet printhead might be in the form of a voltage pulse of a certain amplitude and duration given to each printing pixel. Such voltage pulses are shown schematically in FIG. 4A. Also, by way of example, the media modulation function applied to the media might constitute heat energy in the case of thermal printing or ink drops in the case of ink jet printing. The transfer functions in these examples might describe the amount of heat delivered per volt of actuation in the case of thermal printing, or the number of ink drops delivered to the media per volt of actuation in the case of ink jet printing.
As is common in the art, the actuation function for each printing pixel is timed so as to account for the position of the printhead printing pixels in relation to the media printed pixels where the media modulation was desired to be applied. For simplicity, it is assumed that the printing pixels of a printhead segment form a line perpendicular to the direction of motion of the media sheet. Therefore, the actuation function shown in FIG. 4A corresponds to simultaneous voltage pulses applied to the printing pixels of the printhead to print a line on the media sheet. Other possibilities, such as angulation of the head are well know in the art and require different actuation timing schemes.
The actuation function shown in FIG. 4A corresponds to printing of a uniform line on the media sheet, but this is not the most general case. In general, it is desired to vary the optical density produced on the media sheet. The type of variation possible depends on the printing means. Some printing means, such as thermal printing, have extensive grayscale capability in the sense that the actuation function of the printhead typically has many possible values, corresponding to production by the printhead of many values for the media modulation function, resulting in the creation of pixels on the media with a corresponding range of optical densities. In the case of thermal printing, the printhead activation function (voltage) is varied to produce many levels for the value of the media modulation function (heat applied by the printhead pixel to a donor transfer medium) resulting in many values for the optical density of each printed pixel in the image plane. In other printing means, such as thermal ink jet printing, it is well known in the art that the amount of dye or ink transferred from any one printhead nozzle upon activation onto the image plane cannot be substantially varied. Such printing means are said to have no grayscale capability or very limited grayscale capability.
Actuation, media modulation, and transfer functions for selected printing means discussed below are illustrated in FIGS. 4B-4D. In the case of printers having grayscale capability at each printing pixel, such as thermal printers, a typical actuation function might look like that shown in FIG. 4B, which shows voltages of various amplitudes applied to the printing pixels of a printhead. The media modulation function applied to the media sheet by any printing pixel is varied by varying the actuation function of the corresponding printing pixel. On the other hand, for printers with printing pixels having little or no grayscale capability, a typical actuation function might look like that shown in FIG. 4C, which shows voltages of amplitudes ONE or ZERO applied to the printing pixels of a printhead.
The ratio of the modulation function applied to the media sheet by a particular printing pixel to the actuation function applied to the corresponding printhead printing pixel is the printing pixel transfer function. In the case of thermal printing, the transfer function is given primarily by the amount of heat energy applied by a resistive element to the media sheet for a given level of printhead actuation voltage. For an ink jet printer, the transfer function is primarily given by the amount of ink ejected from a nozzle and, to a lesser extent, by the drop-paper interaction. (In each case, as is well known in the art, effects such as the duration of the voltage pulse may also determine the transfer function).
As generally practiced, the transfer functions of all printing pixels with or without grayscale capability in a printhead are made as uniform as possible to simplify printing and lead manufacturing. Such a uniform transfer function is shown schematically in FIG. 4D, and would apply equally well to actuation function 4B (grayscale) or actuation function 4C (no grayscale).
Uniformity of the printhead transfer function for a pagewidth thermal printhead is highly desired and is reflected in the tight specifications for manufacturing variations between printing pixel resistive elements. Likewise in conventional ink jet pagewidth printheads, nozzles are uniform and the droplets of ink deposited from nozzles in a given printhead are substantially uniform. As is well known in the art, care is taken in the manufacture of such printhead segments to ensure uniformity.
In a printhead for which at most only one printing pixel contributes to a given media sheet printed pixel, the modulation function applied to the media sheet at a given printed pixel is approximated by the product of the actuation function for that pixel multiplied by the transfer function for that printing pixel.
For pagewidth printheads comprised of multiple overlapping printhead segments, the modulation function applied to the media sheet at any given printed pixel is approximated by summing the product of the actuation function and transfer functions for the printing pixels of any printhead segments that contribute to the particular media printed pixel. As is well know in the art of ink jet printheads, some corrections may be needed in this calculation due to the size of ink drops or the time delay between ink drops from different printhead segments. This is illustrated in FIGS. 4E-4I which shows two printhead segment actuation functions, transfer functions, and the resultant modulation function applied to the media sheet, for a case of perfect alignment of the printing pixels of the printhead segments.
If the alignment of the two printhead segments is not perfect, it is possible that a printed pixel on the image plane in the region of overlap may receive an ink droplet from each of two printhead segments, resulting in an undesirable non-uniformity or banding in the region of overlap, as will next be described.
The printing by two adjacent printhead segments that have a single-pixel overlap, caused for example by misalignment, is graphically represented by actuation, transfer, and media modulation functions of FIGS. 5A-5E corresponding to FIGS. 4E-4I (no misalignment), respectively. Again, the total modulation function applied to the media sheet is the sum of the actuation functions multiplied by the transfer functions of all the segments in the printhead array. FIG. 5E illustrates the modulation function applied to the media sheet by the modulation functions of FIGS. 5A and 5B and transfer functions of FIGS. 5C and 5D. Note that a dark band will result at the overlap, as indicated by the positive spike in the modulation function of FIG. 5E.
Printing by two adjacent printhead segments that have a single-pixel gap is graphically represented by modulation functions of FIGS. 6A-6E similar to FIGS. 5A-5E, respectively. Once again, the total modulation function applied to the media sheet is the sum of the actuation functions multiplied by the transfer functions of all the segments in the printhead array. FIG. 6E illustrates the modulation function applied to the media sheet by the modulation functions of FIGS. 6A and 6B and the transfer functions of FIGS. 6C and 6D. Note that a light band will result at the gap, as indicated by the negative spike in the modulation function of FIG. 6E.